It is well known that stabilizers are necessary ingredients in cosmetic formulations. For example, without adequate stabilizers, lipsticks are prone to creeping and melting. While some effort has been spent in identifying improved fundamental materials such as emulsion stabilizers (see, e.g., U.S. Pat. No. 5,871,756), most of the effort in cosmetic stability improvement has been directed toward transfer- and wear-resistance enhancements. U.S. Pat. No. 6,227,358, for example, describes a lipstick composition in which film-forming polymers cross-link after application to the lips to form a semi-permanent, transfer- and wear-resistant film. There has also been a substantial amount of work in which small particles are added to formulations to improve transfer- and wear-resistance. One such formulation, described in U.S. Pat. No. 6,315,990, uses fluorosilane-coated pigment or filler particles to improve wear resistance and waterproofing properties of a variety of cosmetic compositions. Similarly, U.S. Pat. No. 6,254,876 discloses the use of a dispersion of surface-stabilized polymer particles to produce water-, wear- and transfer-resistant cosmetic compositions.
One of the drawbacks of many conventional cosmetic formulations that has not been adequately addressed is the susceptibility to melting at high temperatures. Because of this susceptibility, it is necessary for consumers to protect cosmetic products, especially lipsticks, from exposure to the high temperatures that can be reached, for example, in an enclosed car or in direct sunlight. Conventional approaches to increasing the melting point of lipsticks generally involve producing formulations with a greater proportion of high-melting waxes. Unfortunately, lipsticks with high wax contents have an aesthetically undesirable harsh feel when applied to the lips.
Jojoba is a common ingredient in cosmetics, and because of its emollient properties, is often used in skin lotions, shampoos and hair conditioners (see, e.g., the websites of Crabtree and Evelyn, Ltd. (Woodstock, Conn.) and Nature's Gate Natural Health and Beauty Products (Chatsworth, Calif.) U.S. Pat. Nos. 5,968,530 and 6,280,746 describe jojoba ester-based compositions that serve as emollient additives for a variety of pigment-containing cosmetic products. Barone, et al., in “Lipstick Technology,” Monograph Number 8, Society of Cosmetic Chemists (2002) also disclose the use of jojoba esters as lipstick additives that provide superior emolliency, improved stick structure, and are oxidatively stable. Jojoba, a natural product derived from the jojoba tree, comprises a mixture of long-chained linear esters with a combination of mixed tocopherols, free sterols and other unsaponifiable material making up the balance. Jojoba esters exhibit stability at high temperatures, and as such, are known additions to cosmetic compositions to increase thermostability. See e.g., “Cosmetic Chemistry of Natural Jojoba” available from the website of Purcell Jojoba International (Avila Beach, Calif.), and U.S. Pat. No. 5,538,718, describing a cosmetic stick composition in which jojoba, or one of a variety of other naturally regenerable vegetable waxes and oils, is substituted for conventional high-melting paraffin waxes. Jojoba esters are also able to resist hydrolysis and oxidation and thus have become a common replacement for conventional lipids such as triglyceride oils in cosmetic compositions. Jojoba esters thus offer cosmetic chemists a natural cosmetic lipid with excellent emolliency, moisturization and oxidative stability. Cosmetic compositions containing jojoba esters, however, are prone to melting and sweating at high temperatures.
Accordingly, there is a need in the industry for heat-resistant cosmetic compositions that are oxidatively stable and that have a pleasant aesthetic quality when applied to the skin.